The present invention relates to a constant-voltage switching power supply provided with an overvoltage output protecting circuit, and relates to and an electronic apparatus provided with an overvoltage protecting circuit.
A constant-voltage switching power supply rarely generates unnecessary power as heat from a transistor since a switching transistor therein only repeats the ON-OFF switching operation. The constant-voltage switching power supply uses a compact and small-loss high-frequency transformer. Thus the constant-voltage switching power supply is advantageous in that the power loss of the entire circuit can be made small. FIG. 3 is a circuit diagram showing an example of a related-art constant-voltage switching power supply.
A smoother 1 comprising a rectifier bridge DB1 and a capacitor C11 smoothes the AC voltage supplied from an AC power source Vin to convert the AC voltage to a DC voltage. The DC voltage is switched by a switching transistor (field-effect transistor) Q1 and converted to a high-frequency pulse. The high-frequency pulse is transformed by a high-frequency transformer T1 then converted to a DC voltage again by a high-frequency rectifier 2 and outputted across a Vout terminal and a GND terminal. In a case where there is a variation in the output voltage, the voltage comparator/detector 3 detects a variation in the voltage and notifies that to a duty ratio controller 4 via a photocoupler PC1. The duty ratio controller 4 changes the ON-OFF interval of the switching transistor Q1 to control the duty ratio of the high-frequency pulse. The average voltage of the high-frequency pulse becomes the DC output voltage and the output voltage is controlled by the duty ratio. Control of the duty ratio of the high-frequency pulse is made so that, when the DC output voltage is higher, the ON duty will be smaller and when the DC output voltage is lower, the ON duty will be greater.
In the related-art constant-voltage switching power supply of this configuration; an AC voltage supplied from the AC power source Vin is converted to a DC voltage by the smoother 1, and the DC voltage causes a current to flow through an activation resistor R1, thus elevating the gate voltage of the switching transistor Q1. This turns on the switching transistor Q1 and generates a voltage on a first primary coil of the high-frequency transformer T1 and a phase-inverted voltage corresponding to the number of turns on a second primary coil P2. The voltage generated in the second primary coil applies a positive feedback on the gate of the switching transistor Q1 via a capacitor C1 and a resistor R2. The base of the transistor Q2 is charged by a coupler current of a photocoupler PC1 which feeds back a variation in the DC output voltage and a current flowing through a Zener diode ZD1.
When a current flows through the first primary coil P1, a current attempts to flow through the secondary wiring S1 of the high-frequency transformer T1. The diode D1 blocks the current so that the corresponding energy is stored in the high-frequency transformer T1. When the base voltage of the transistor Q1 reaches the ON voltage, the switching transistor Q1 turns off, causing the energy to be transmitted from the secondary wiring S1. On the second primary coil P2 is applied a reverse bias thus causing the base of the transistor Q2 to be discharged. When all the energy stored in the high-frequency transformer T1 is generated from the secondary wiring S1, the switching transistor Q1 starts to turn on again with a counterelectromotive force.
A high-frequency pulse is generated by repeating the above operation. Then the transistor Q2 is ON-OFF controlled by the coupler current of the photocoupler PC1 which feeds back a variation in the DC output voltage. This causes ON-OFF control of the transistor Q1, which controls the duty ratio of the high-frequency pulse. Thus a voltage specified by a Zener diode ZD2 is stably outputted at the DC output terminal Vout.
An overvoltage detector 26 detects whether the DC output voltage defined by a Zener diode ZD3 is overvoltage, and transmits the overvoltage thus detected to a switching transistor deactivator 5 through a photocoupler PC2. In the overvoltage detector 26, when the DC output voltage becomes an overvoltage, the photocoupler PC2 in the overvoltage detector 26 is turned ON to operate the switching transistor deactivator 5 by the coupler current of the photocoupler PC2 in the switching transistor deactivator 5. The switching transistor deactivator 5 turns OFF the switching transistor Q1, thereby stopping the switching operation of the switching transistor Q1.
Accordingly, when the photocoupler PC1 of the voltage comparator/detector 3 breaks down so that constant-voltage control is disabled and the DC output voltage is abnormally raised to be an overvoltage, for example, the overvoltage detector 26 can detect an overvoltage to stop the operation of the constant-voltage switching power supply, thereby preventing heat from being generated by the overvoltage.
However, the voltage comparator/detector 3 and the overvoltage detector 26 have such circuit configurations as to depend on the photocouplers provided across the primary and secondary sides of the high-frequency transformer T1. For this reason, when some abnormality is occurred on the secondary side of the high-frequency transformer T1 by the breakdown of a load connected to the secondary side of the high-frequency transformer T1, there is an anxiety that both the photocoupler PC1 and the photocoupler PC2 might break down. Consequently, there is an anxiety that neither the duty ratio controller 4 nor the switching transistor deactivator 5 on the primary side of the high-frequency transformer T1 might function and the control of the switching transistor Q1 might be perfectly disabled to cause the DC output voltage to be the overvoltage.
For another related art, an overvoltage protecting circuit is disclosed in Japanese Patent Publication No. 8-256474A. The overvoltage protecting circuit relates to a self-excitation type switching power source and has an object to protect a load from an abnormal rise in an output voltage which is caused by the breakdown of a power supply. The overvoltage protecting circuit is provided with: an ON period limiter for limiting the ON period of a switching transistor; a negative rectifier for taking a negative voltage from the feedback coil of a switching transformer; and a detector for detecting whether the output voltage of the negative rectifier is a predetermined value or less. The overvoltage protecting circuit is so configured that the output voltage of the negative rectifier is also dropped proportionally when the output voltage is raised, and a detection signal is transmitted from the detector to the ON period limiter in a case where the output voltage becomes the predetermined value or less, so that the ON period of the switching transistor is thus limited. Also in this overvoltage protecting circuit, the overvoltage problem as explained the above might be occurred.
In a general electronic apparatus comprising a power supply device such as a constant-voltage switching power supply or an electronic apparatus having such a structure that a power supply device is not provided therein but a power is supplied from an external power supply device such as an AC adapter, an overcorrent might flow to a part of circuits to damage them and a part of the circuits might generate heat to have a high temperature when an abnormality occurs on the power supply device and a much higher voltage than a rated voltage is applied to the circuits in the electronic apparatus.
For this reason, a general power supply device is provided with an output overvoltage protecting circuit for preventing an output voltage from becoming an overvoltage. On the other hand, there is also an anxiety that an overvoltage might be generated in a part of the circuits in the electronic apparatus by the abnormality occurred on the circuits in the electronic apparatus even if the power supply device is normally operated. Also in such a case, similarly, there is an anxiety that an overcorrent might flow to a part of the circuits to damage them and a part of the circuits might generate heat to have a high temperature. In the general electronic apparatus, therefore, an overvoltage protecting circuit for preventing the voltage of the circuit in the electronic apparatus from being a predetermined value or more is provided separately from the output overvoltage protecting circuit of the power supply device.
For example, Japanese Patent Publication No. 2001-5576A discloses an overvoltage protecting circuit for clamping the voltage of a circuit in an electronic apparatus by a Zener diode to prevent an overcorrent from flowing to a part of the circuits in the electronic apparatus, thereby preventing the circuits from being broken and preventing heat generation from being caused in the circuits in the electronic apparatus.
In such a configuration, since a power is continuously supplied from a power supply device, an overvoltage is clamped by the Zener diode so that a large current might continuously flow to the Zener diode to generate heat. In the case where a larger current than a rated current flows to the Zener diode, the Zener diode is broken down. In many cases, the breakdown of the Zener diode caused by such an overcurrent brings a short-circuit breakdown, that is, a short-circuited state. Consequently, an output overcurrent is detected on the power supply device side so that the power supply device is stopped and both the power supply device and the electronic apparatus are protected from an overvoltage.
However, in the case where the Zener diode causes an open breakdown, there is an anxiety that the voltage in the electronic apparatus cannot be clamped to generate an overvoltage so that an overcorrent might flow to a part of the circuits. Therefore, the circuits might be broken down, and at the same time, a part of the circuits might generate heat to have a high temperature.
Moreover, many of general electronic apparatuses have such a structure that a DC voltage supplied from a power supply device is converted depending on each of circuits in the electronic apparatus by a voltage output circuit such as a DC-DC converter. For example, Japanese Patent Publication No 2003-72057A discloses a circuit for ON/OFF controlling the output voltage of a DC-DC converter. In such a configuration, the output voltage of a voltage output circuit such as a DC-DC converter is OFF controlled to prevent an overcorrent from flowing to a part of the circuits in the electronic apparatus, thereby preventing the circuit from being broken and preventing heat generation from being caused in the circuits in the electronic apparatus.
However, since the power is continuously supplied from the power supply device also after the overvoltage is detected in the electronic apparatus in the same manner as explained the above, in the case where an abnormality is occurred in the voltage output circuit such as a DC-DC converter, there is an anxiety that an overcorrent might flow to a part of the circuits and the circuits might be thereby broken, and at the same time, a part of the circuits might generate heat to have a high temperature.